Hydroponics is a type of horticulture and a subset of hydroculture, which is a method of growing plants, usually crops, without soil, by using mineral nutrient solutions in an aqueous solvent. Terrestrial plants may be grown with only their roots exposed to the nutritious liquid, or, in addition, the roots may be physically supported by an inert medium such as perlite, gravel, or other substrates. Despite inert media, roots can cause changes of the rhizosphere pH and root exudates can affect the rhizosphere biology.The nutrients used in hydroponic systems can come from many different sources, including (but not limited to) fish excrement, duck manure, purchased chemical fertilizers, or artificial nutrient solutions.Plants commonly grown hydroponically, on inert media, include tomatoes, peppers, cucumbers, strawberries, lettuces, marijuana, and model plants like Arabidopsis thaliana.Hydroponics offers many advantages, one of them being a decrease in water usage for agriculture. To grow 1 kilogram (2.2 lb) of tomatoes using intensive farming methods requires 400 liters (88 imp gal; 110 U.S. gal) of water; using hydroponics, 70 liters (15 imp gal; 18 U.S. gal); and only 20 liters (4.4 imp gal; 5.3 U.S. gal) using aeroponics. Since it takes much less water to grow produce, it could be possible in the future for providers in harsh environments with little accessible water to grow their own food.